The present invention relates to a brake system for automotive vehicles comprising a braking pressure generator and a controllable braking pressure modulator which is arranged in the pressure medium conduits leading to the rear-wheel brakes. The system allows variation of the apportioning of the braking pressure onto the wheel brakes of the front axle and the rear axle for the purpose of approximation to an ideal brake force distribution dependent upon the instantaneous axle load.
It is known that the dimensioning of brake systems is rendered difficult by the changes in static and dynamic axle loads and by axle load shifts. Therefore, by invariably adjusting the braking pressure distribution onto the front axle and the rear axle, optimum conditions may be accomplished at most in a specific state of loading and driving.
Moreover, for decreasing the imminent danger of skidding, the share in braking pressure allotted to the rear-wheel brakes is desired to be as small as to allow the rear wheels to lock in all situations only after the front wheels do, although the rear wheels are relieved from load owing to the dynamic axle load shift when braking at higher speed and, in consequence thereof, tend to lock sooner than the front wheels. For this reason, a considerably larger portion of braking pressure must be allotted to the front axle when brake force distribution is invariable.
A certain improvement or, respectively, approximation to the ideal brake force distribution permits to be obtained by the known brake force distributors which vary the brake force distribution onto the front and the rear axle according to various function principles in response to braking pressure, load or deceleration. The extent of adaptation to the ideal distribution attainable differs widely. In general, even when sophisticated devices and great adjusting efforts are involved, satisfactory adaptation to the ideal characteristic curve can be obtained at most in either one of the two borderline cases "unloaded/loaded".
A brake force distributor is known in the prior art wherein the static axle load distribution is measured by sensors when the vehicle is at standstill. The results thereof are delivered to a microcomputer which governs the brake force distribution, in consideration of the test values, according to a memorized mathematical relation and while additionally considering the pressure measured in the front-axle circuit and in the rear axle circuit (European publication EP-AL No. 062246). Despite the high effort entailed, in this device the adaptation is also far from being ideal in the majority of driving situations, because only a theoretical adherence value, but not the actual value of adherence between road and tires can be made the basis for dimensioning the brakes, and for reasons of safety and for prevention of overbraking of the rear wheels, the device must be dimensioned such that the front axle supplies the greater contribution to braking.
It has been proposed to have electromagnetically controlled modulators control the brake slip at the rear wheels in dependence on the brake slip at the front wheels such that a value of adherence will result at the rear axle which is always equal to, or somewhat less than, that at the front wheels (German publication No. P 33 01 948).
It is an object of the present invention to overcome the disadvantgages described and to create a brake system which permits to attain a better approximation to the ideal brake force distribution or, respectively, brake force apportioning, onto the front and the rear axle without impairing the directional stability of the vehicle and, respectively, without increasing the imminent danger of skidding (due to locking rear wheels).